Protein kinases are a family of enzymes that catalyse the phosphorylation of specific residues in proteins. In general protein kinases fall into several groups; those which preferentially phosphorylate serine and/or threonine residues, those which preferentially phosphorylate tyrosine residues and those which phosphorylate both tyrosine and Ser/Thr residues. Protein kinases are therefore key elements in signal transduction pathways responsible for transducing extracellular signals, including the action of cytokines on their receptors, to the nuclei, triggering various biological events. The many roles of protein kinases in normal cell physiology include cell cycle control and cell growth, differentiation, apoptosis, cell mobility and mitogenesis.
Protein kinases include, for example, but are not limited to, members of the Protein Tyrosine Kinase family (PTKs), which in turn can be divided into the cytoplasmic PTKs and the receptor PTKs (RTKs). The cytoplasmic PTKs include the SRC family (including: BLK; FGR; FYN; HCK; LCK; LYN; SRC; YES and YRK); the BRK Family (including: BRK; FRK, SAD; and SRM); the CSK family (including: CSK and CTK); the BTK family (including BTK; ITK; TEC; MKK2 and TXK), the Janus kinase family (including: JAK1, JAK2, JAK3 and TYK2); the FAK family (including FAK and PYK2); the Fes family (including FES and FER), the ZAP70 family (including ZAP70 and SYK); the ACK family (including ACK1 and ACK2); and the Abl family (including ABL and ARG). The RTK family includes the EGF Receptor family (including, EGFR, HER2, HER3 and HER4); the Insulin Receptor family (including INS R and IGF1 R); the PDGF Receptor family (including PDGFRα, PDGFRβ, CSF1R, KIT, FLK2); the VEGF Receptor family (including; FLT1, FLK1 and FLT4); the FGF Receptor family (including FGFR1, FGFR2, FGFR3 and FGFR4); the CCK4 family (including CCK4); the MET family (including MET and RON); the TRK family (including TRKA, TRKB, and TRKC); the AXL family (including AXL, MER, and SKY); the TIE/TEK family (including TIE1 and TIE2/TEK); the EPH family (including EPHA1, EPHA2, EPHA3, EPHA4, EPHA5, EPHA6, EPHA7, EPHA8, EPHB1, EPHB2, EPHB3, EPHB4, EPHB5, EPHB6); the RYK family (including RYK); the MCK family (including MCK and TYRO10); the ROS family (including ROS); the RET family (including RET); the LTK family (including LTK and ALK); the ROR family (including ROR1 and ROR2); The Musk family (including Musk); the LMR family including LMR1, LMR2 and LMR3); and the SuRTK106 family (including SuRTK106).
Similarly, the serine/threonine specific kinases comprise a number of distinct sub families, including; the extracellular signal regulated kinases (p42/ERK2 and p44/ERKI); c Jun NH2 terminal kinase (JNK); cAMP responsive element binding protein kinases (CREBK); cAMP dependent kinase (CAPK); mitogen activated protein kinase activated protein kinase (MAPK and its relatives); stress activated protein kinase p38/SAPK2; mitogen and stress activated kinase (MSK); protein kinases, PKA, PKB and PKC inter alia.
Additionally, the genomes of a number of pathogenic organisms possess genes encoding protein kinases. For example, the malarial parasite Plasmodium falciparum and viruses such as HPV and Hepatitis viruses appear to bear kinase related genes.
Inappropriately high protein kinase activity has been implicated in many diseases resulting from abnormal cellular function. This might arise either directly or indirectly, for example by failure of the proper control mechanisms for the kinase, related for example to mutation, over expression or inappropriate activation of the enzyme; or by over or under production of cytokines or growth factors also participating in the transduction of signals upstream or downstream of the kinase. In all of these instances, selective inhibition of the action of the kinase might be expected to have a beneficial effect.
Diseases where aberrant kinase activity has been implicated include: diabetes; restenosis; atherosclerosis; fibrosis of the liver and kidney; ocular diseases; myelo and lymphoproliferative disorders; cancer such as prostate cancer, colon cancer, breast cancer, head and neck cancer, leukemia and lymphoma; and, auto immune diseases such as Atopic Dermatitis, Asthma, rheumatoid arthritis, Crohn's disease, psoriasis, Crouzon syndrome, achondroplasia, and thanatophoric dysplasia.
The JAK family of protein tyrosine kinases (PTKs) play a central role in the cytokine dependent regulation of the proliferation and end function of several important cell types of the immune system. (reviewed in Kisseleva et al 2002)
The central role played by the JAK family of protein tyrosine kinases in the cytokine dependent regulation of the proliferation and end function of several important cell types means that agents which inhibit JAK are useful in the prevention and chemotherapy of disease states dependent on these enzymes. Potent and specific inhibitors of each of the currently known four JAK family members will provide a means of inhibiting the action of those cytokines that drive immune pathologies, such as asthma (e.g. IL 13; JAK1, TYK2 and JAK2), leukemia/lymphoma (e.g. IL 2: JAK1 and JAK3) and myeloproliferative syndromes such as Polycythemia vera (Takemoto, S et al, 2002; El-Adawi, H. et al. 2003, Booz, G. W., Day, J. N., Speth, R. & Baker, K. M., 2002; James, C. et al., 2005). Furthermore, certain types of cancer such as prostate cancer develop autocrine production of certain cytokines as a selectable mechanism of developing growth and/or metastatic potential. An example of this is cancer of the prostate, where IL 6 is produced by and stimulates the growth of prostate cancer cell lines such as TSU and TC3 (Spiotto M T, and Chung T D, 2000). Interestingly, levels of IL 6 are elevated in sera of patients with metastatic prostate cancer.
A direct comparison of the four currently known mammalian JAK family members reveals the presence of seven highly conserved domains (Harpur et al, 1992). In seeking a nomenclature for the highly conserved domains characteristic of this family of PTKs, the classification used was guided by the approach of Pawson and co workers (Sadowski et al, 1986) in their treatment of the SRC homology (SH) domains. The domains have been enumerated accordingly with most C terminal homology domain designated JAK Homology domain 1 (JH1). The next domain N terminal to JH1 is the kinase related domain, designated here as the JH2 domain. Each domain is then enumerated up to the JH7 located at the N terminus. The high degree of conservation of these JAK homology (JH) domains suggests that they are each likely to play an important role in the cellular processes in which these proteins operate. However, the boundaries of the JAK homology domains are arbitrary, and may or may not define functional domains. Nonetheless, their delineation is a useful device to aid the consideration of the overall structural similarity of this class of proteins
The feature most characteristic of the JAK family of PTKs is the possession of two kinase related domains (JH1 and JH2) (Wilks et al, 1991). The putative PTK domain of JAK1 (JH1) contains highly conserved motifs typical of PTK domains, including the presence of a tyrosine residue at position 1022 located 11 residues C terminal to sub domain VII that is considered diagnostic of membership of the tyrosine specific class of protein kinases. Alignment of the human JAK1 PTK domain (255 amino acids), with other members of the PTK class of proteins revealed homology with other functional PTKs (for example, 28% identity with c-fes (Wilks and Kurban, 1988) and 37% homology to TRK (Kozma et al, 1988). The JH1 domains of each of the JAK family members possess an interesting idiosyncrasy within the highly conserved sub domain VIII motif (residues 1015 to 1027 in JAK2) that is believed to lie close to the active site, and define substrate specificity. The phenylalanine and tyrosine residues flanking the conserved tryptophan in this motif are unique to the JAK family of PTKs. Aside from this element, the JH 1 domains of each of the members of the JAK family are typical PTK domains Hanks S K, Hunter T 1995 and contain the conserved structural features: N-terminal lobe, C-terminal lobe glycine-rich/nucleotide binding loop, catalytic loop, activation loop and sets of other amino acids composing the catalytic domain of kinases.
The delineation of a particularly elegant signal transduction pathway downstream of the non-protein tyrosine kinase cytokine receptors has recently been achieved. In this pathway the key components are: (i) A cytokine receptor chain (or chains) such as the Interleukin 4 receptor or the Interferon γ receptor; (ii) a member (or members) of the JAK family of PTKs; (iii) a member(s) of the STAT family of transcription factors, and (iv) a sequence specific DNA element to which the activated STAT will bind. In addition, other effectors and regulators can contribute to JAK/STAT pathway signaling events (reviewed in Rawlings et al, 2004) including, SOCS (suppressors of cytokine signaling), PTPs (protein tyrosine phosphatases), STAMs (signal-transucing adaptor molecules), StIPs (stat-interacting proteins) and adapters of the SH2B/Lnk/APS family.
A review of the JAK/STAT literature offers strong support to the notion that this pathway is important for the recruitment and marshalling of the host immune response to environmental insults, such as viral and bacterial infection. This is well exemplified in Table 1. Information accumulated from gene knock-out experiments have underlined the importance of members of the JAK family to the intracellular signalling triggered by a number of important immune regulatory cytokines. The therapeutic possibilities stemming from inhibiting (or enhancing) the JAK/STAT pathway are thus largely in the sphere of immune modulation, and as such are likely to be promising drugs for the treatment of a range of pathologies in this area. In addition to the diseases listed in Table 1, inhibitors of JAKs could be used as immunosuppressive agents for organ transplants and autoimmune diseases such as lupus, multiple sclerosis, rheumatoid arthritis, Type I diabetes, autoimmune thyroid disorders, Alzheimer's disease and other autoimmune diseases. Additionally, treatment of cancers such as prostate cancer by JAK inhibitors is indicated.
TABLE 1Cell TypesDisease TypeInvolvedCharacteristicsAtopyAllergic AsthmaMast CellsT-cell activation ofAtopic Dermatitis (Eczema)EosinophilsB-cells followed byAllergic RhinitisT-CellsIgE mediatedB-Cellsactivation ofresident Mast cells andEosinophilsCell MediatedHypersensitivityAllergic Contact DermatitisT-cellsT-cell hypersensitivityHypersensitivity PneumonitisB-cellsRheumatic DiseasesSystemic LupusMonocytesCytokine ProductionErythematosus(e.g. TNF, IL-1,(SLE)CSF-1, GM-CSF)Rheumatoid ArthritisMacrophagesT-cell ActivationJuvenile ArthritisNeutrophilsJAK/STAT activationSjögren's SyndromeMast CellsSclerodermaEosinophilsPolymyositisT-CellsAnkylosing SpondylitisB-CellsPsoriatic ArthritisViral DiseasesEpstein Barr Virus (EBV)LymphocytesJAK/STAT ActivationHepatitis BHepatocytesJAK/STAT ActivationHepatitis CHepatocytesJAK/STAT InhibitionHIVLymphocytesJAK/STAT ActivationHTLV 1LymphocytesJAK/STAT ActivationVaricella-Zoster Virus (VZV)FibroblastsJAK/STAT InhibitionHuman Papilloma VirusEpithelial cellsJAK/STAT Inhibition(HPV)CancerLeukemiaLeucocytes(Cytokine productionLymphomaLymphocytes(JAK/STATActivationNeurodegenerative DiseasesMotor Neuron DiseaseNeuronsMutated SOD1Cardiovascular DiseasesAtherosclerosis &LymphocytesJAK/STAT ActivationArteriosclerosisMacrophagesJAK/STAT ActivationMyoepithelialcellsCardiac HypertrophyCardiac MyocytesJAK/STAT ActivationIschemiaCardiac MyocytesJAK/STAT ActivationPulmonary HypertensionLung EpitheliumJAK/STAT Activation